The present invention is directed to a method for manufacturing a catalytic converter.
DE 16 42 921 C3 teaches a method of placing a honey-like suspension on spherical steatite carriers. The suspension contains a solvent and a mixture of finely-divided suspended titanium dioxide and a dissolved vanadium compound or dissolved vanadium pentoxide. The spheres may be primed, placed in a heated container (e.g., a coating drum), and sprayed with the suspension while the spheres are moved in the vessel. The solvent evaporates from the suspension during this process leaving behind a dried layer containing vanadium pentoxide and titanium dioxide.
It is also known that catalytic converters (e.g., exhaust catalytic converters for motor vehicles or chemical reactors) can be made by coating a substrate with a catalyst material. A basic body is contacted with a slurry for example, which contains fine-grained catalytically active material and an organic or inorganic solvent (e.g., by dipping or wetting, or rolling in a powdered catalyst material in the moistened state). Methods of this type are described in DE 29 47 702 A1 and DE 25 26 238 A1. To produce sufficient adhesion between the catalyst material and the substrate, it is known that the substrate can be provided with an adhesion promoter layer before coating. The thickness of this adhesion layer is generally approximately 10 .mu.m. Without an adhesion promoter, adhesion of the catalyst layer is often insufficient. The solvent is then released by heating the coated substrate.
Although the known methods are technically simple, the catalyst layers produced have a number of disadvantages.
Coating is completely nonselective with regard to the coating location and the quantity of catalyst material applied to the substrate. To produce sufficient adhesion between the coating and the substrate, it is often necessary to apply an additional adhesion promoter or a primer (e.g., a wash coat), before the coating. Nevertheless, adhesion under operating conditions is often defective despite this adhesion promoter. In dip-coated exhaust gas catalytic converters for automobiles, significant losses of catalyst material are observed after a fairly lengthy operating time. The catalyst layer erodes during operation. Further, such dipped layers are not impact-proof. If they are subjected to mechanical impacts, the catalyst material separates from the substrate. Also, the adhesion promoter layer impairs the heat bond between the catalyst material and the substrate.
Moreover, not every substrate can be coated. A metallic steel substrate generally oxidizes before the coating process at high temperatures. Aluminum-containing steel, for example, forms a thick layer of aluminum oxide at the surface. The rough oxidized surface that forms improves the adhesion of the layer. However, this cannot be used with aluminum-free stainless steel (VA).
When the basic bodies forming the substrate are uneven or structured, it is practically impossible to create a uniform layer thickness because wetting of the substrate depends on its geometry and on the surface tension of the coating material. Even with flat substrates, however, the thickness of the coating is insufficiently uniform because the thickness of dip-coats increases outward toward the edge. Significant inhomgeneity of the coating is observed not only in the lateral direction but also in the vertical direction. Larger-diameter or heavier grains sink as the material dries, making the particle-size distribution in the deposited layer fundamentally nonuniform. Nor is it possible to establish a concentration profile of the catalytically active material along a lengthwise direction of the substrate.
The object of the present invention is to provide a method for manufacturing a catalytic converter that makes it possible to deposit a catalyst material with a large surface area and good adhesion on a substrate, and which dispenses with additional adhesion promoter layers.
This object is achieved in a method according to the present invention in which a catalytic converter is made by spraying a layer of catalyst material onto a metal substrate using a sprayer. For this purpose, the catalyst material is mixed with a solvent, preferably a solvent that forms a suspension with the catalyst. The catalyst material is then sprayed onto the substrate, which has been heated to a preset temperature. The catalyst material is present in the suspension as carrier particles coated with catalytically active material.
The method according to the present invention makes it possible to manufacture a catalytic converter distinguished by contour-conforming coating of a substrate and also makes it possible to differentiate the layer locally in terms of its geometric and chemical properties. This makes it simple to establish a concentration gradient of the catalytically active material and locally differentiated coating in broad areas. The catalytic converter manufactured according to the present invention is also characterized by significantly improved adhesion of the coating to the substrate and improved erosion resistance by comparison to catalytic converters made by the traditional dip process. In particular, the layers deposited according to the present invention are impact resistant. It is also possible to solder sheet metal coated according to the present invention at temperatures of approximately 1000.degree. C. without the catalyst layer flaking off. Further, since adhesion promoter layers are unnecessary, the heat bond between the catalyst material and the substrate is very good.
Preferably, the substrate temperature during spraying is set so that the solvent evaporates rapidly when the sprayed material touches the substrate. This fixes the coating material locally so that it can no longer flow and creep on the surface of the substrate. At the same time, separation of the coating according to the size and/or weight of the particles is prevented.
Preferably, the solvent is polar. More preferably, the solvent is water. The advantage of this is that water is a medium that is simple and nonproblematical to handle.
It is particularly advantageous to move the substrate and the sprayer relative to one another, preferably parallel to one another. The movement can be adjusted such that a homogenous layer of uniform thickness or a layer with a specific change in thickness is sprayed. It is also possible for the substrate to be provided (and coated) with a mask, particularly a perforated mask, so that the coating on the substrate is structured.
One advantageous method is to produce a catalyst concentration profile by dividing the areas of the substrate to be coated into regions that can be individually and differently coated and which differ from each other in the concentration and/or layer thickness and/or type and/or grain and/or porosity of the coating of the catalyst material.
Another advantageous embodiment of the method comprises coating the substrate in several sequential steps. For one thing, the layer thickness can be defined in this way. For another, in an improvement on the method, the composition and/or the type of the catalyst material can be changed advantageously between two sequential coating steps. This makes it readily possible for the geometric properties and/or chemical activity of the catalytic converter to be matched to widely varying applications.
Substrate bodies with complex structures can be coated true to the contour, and it is also possible to coat large surface areas homogeneously. The substrate to be coated does not have to be absorbent. In particular, it is possible to coat substrates that are nonoxidizable or not readily oxidizable, such as aluminum-free stainless steel (VA), with catalyst material.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.